1. Field of the Invention
This invention relates generally to optical data storage systems and more specifically to a storage system having multiple data storage surfaces.
2. Description of the Prior Art
Optical data storage systems provide a means for storing great quantities of data on a disk. The data is accessed by focussing a laser beam onto the data layer of the disk and then detecting the reflected light beam. Various kinds of systems are known. In a ROM (Read Only Memory) system, data is permanently embedded as marks in the disk at the time of manufacture of the disk. The data is detected as a change in reflectivity as the laser beam passes over the data marks. A WORM (Write-Once Read Many) system allows the user to write data by making marks, such as pits, on a blank optical disk surface. Once the data is recorded onto the disk it cannot be erased. The data in a WORM system is also detected as a change in reflectivity
Erasable optical systems are also known. These systems use the laser to heat the data layer above a critical temperature in order to write and erase the dam. Magneto-optical recording systems record data by orienting the magnetic domain of a spot in either an up or a down position. The data is read by directing a low power laser to the data layer. The differences in magnetic domain direction cause the plane of polarization of the light beam to be rotated one way or the other, clockwise or counterclockwise. This change in orientation of polarization is then detected. Phase change recording uses a structural change of the data layer itself (amorphous/crystalline are two common types of phases) to record the data. The data is detected as changes in reflectivity as a beam passes over the different phases.
In order to increase the storage capacity of an optical disk, multiple data layer systems have been proposed. An optical disk having two or more data layers may in theory be accessed at different layers by changing the focal position of the lens. Examples of this approach include U.S. Pat. No. 3,946,367 issued Mar. 23, 1976 by Wohlmut, et al.; U.S. Pat. No. 4,219,704 issued Aug. 26, 1980 to Russell; U.S. Pat. No. 4,450,553 issued May 22, 1984 to Holster, et al.; U.S. Pat. No. 4,905,215 issued Feb. 27, 1990 to Hattori, et al.; Japanese Published Application, 63-276732 published Nov. 15, 1988 by Watanabe, et al.; and IBM Technical Disclosure Bulletin, Vol. 30, No. 2, p. 667, July 1987, by Arter, et al.
The problem with these prior art systems has been that the ability to clearly read the data recorded is very difficult if there is more than one data layer. The cross-talk signals from the other data layers greatly reduces the ability to read. Also, there are problems in focussing at the different depths and in generating a tracking signal. An optical data storage system is needed which overcomes these problems.